librbd/cache/pwl: generate image cache state json under m_lock

librbd/cache/pwl: generate image cache state json under m_lock

The previous commit moved the entirety of write_image_cache_state()
from under m_lock.  This was a step too far because the generated image
cache state json is no longer guaranteed to be consistent.

Arrange for m_lock to still be held during image cache json generation
but released before owner_lock is grabbed.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>